1. Field of the Invention
This invention relates, generally, to security papers. More specifically, it relates to security papers intended for manufacturing bank notes and a process for manufacturing such papers.
2. Brief Description of the Prior Art
Bank notes are exposed during their service life to much handling and risk of soiling. In order to give them high durability, the fibrous substrate is protected with one or more layers of a protective coating which has a barrier effect against soiling.
The choice of the protective coating is complex, since it must both provide a barrier function and not excessively impair the printability of the paper during the manufacture of the notes.
Many protective coatings have already been disclosed in the prior art.
For example, EPO patent no. EP 514 455 B1 discloses improving the resistance to circulation of bank notes by impregnating the fibrous substrate, before printing, with a composition which contains one or more binders chosen for their mechanical properties.
According to said patent, at least one of the faces of the fibrous substrate is treated with a composition comprising one or more fillers chosen from mineral tillers and/or plastic coating pigments, and at least one elastomeric binder in an amount greater than 25 parts per 100 parts by dry weight of filler. The elastomeric binder may be chosen from the group formed by aqueous dispersions of polyurethane, of acrylate copolymer, of optionally carboxylated styrene-butadiene copolymer, of polymers for which one of the monomers is acrylonitrile or isoprene or neoprene, or mixtures thereof. Preferably, a polyurethane is used. The filler is preferably mineral, chosen from silicas and kaolins. To deposit the coating layer, an air-knife coater is preferred, so as to respect the watermark.
The papers made by using the teaching of said patent are satisfactory overall in the sense that they have good resistance to abrasion of the primed paper and also satisfactory resistance to soiling with respect to greases and pigments in liquid medium. However, their resistance with respect to greases and minerals in pulverulent medium could still be further improved.
Patent application U.S. 2007/0017647 A1 discloses a paper for manufacturing bank notes, which has improved resistance to soiling. According to the teaching of said patent application, the protective coating comprises at least two layers applied successively, including a base layer which is in contact with the fibrous substrate and closes its pores and an outer layer which protects the substrate against physical and chemical influences. The base layer may comprise polyurethane, while the outer layer is preferentially photo-crosslinkable. The application of the base layer may be performed by flexographic printing. In one example, the base layer is an aqueous dispersion of a polyurethane styrene acrylic mixture, while the outer layer is a styrene acrylic aqueous dispersion. When the outer layer is crosslinkable by drying, it may comprise an aqueous dispersion, preferably without polyurethane, for example based on styrene acrylic.
WO 96/28610 discloses the application of a polyurethane-based liquid composition to a fibrous substrate, in order to form a protective coating.
U.S. 2006/0127649 indicates that the application of a liquid composition makes it possible to obtain better resistance to soiling, but does not improve the mechanical stability of the substrate.
Patent application U.S. 2004/0023008 A1 discloses a security paper for manufacturing bank notes. The paper comprises a fibrous substrate which is coated on at least one of its faces with a transparent or translucent protective layer comprising colloidal silica, and at least one transparent or translucent elastomeric binder, for example comprising polyurethane.
Publication WO 2008/054580 A1 discloses a process for giving a fibrous substrate resistance to humidity and to soiling, which consists in impregnating the substrate on two opposite faces with an aqueous composition comprising one or more thermoplastic resins chosen from resins bearing an ester bond, polyurethane resins, functionalized polyurethane and copolymers and mixtures of these resins. More particularly, the resins may be chosen from aqueous dispersions of a polymer, in particular bearing resin particles chosen from polyurethane, polyether-urethane and urethane-acrylic resins and mixtures thereof. In a preferred embodiment, the composition that is applied comprises, besides the aqueous polymer dispersion, one or more pigments such as titanium dioxide and optionally one or more crosslinking agents.
U.S. Pat. No. 1,499,235 discloses the improvement of the resistance to circulation of a bank note by impregnating the fibrous substrate with a latex comprising a copolymer based on carboxylated styrene and butadiene.
Patent EP 815 321 B1 discloses a paper for manufacturing bank notes, in which an aqueous polyurethane dispersion is deposited on the fibrous substrate, this polyurethane dispersion being free of fillers and transparent.
High-durability papers in which a fibrous substrate is coated with a PET film bonded to the substrate via a polyurethane-based adhesive, this PET film itself being covered with an acrylic varnish, are moreover known, as described in publications EP 1 545 902, EP 1 854 641 and EP 1 827 823.
The known solutions are not entirely satisfactory in terms of manufacturing cost and/or performance.
U.S. Pat. No. 3,288,632 discloses a process for manufacturing glazed paper, intended for the manufacture of magazines or books, of high opacity. This process comprises the successive application of two highly pigmented layers. The teaching of said patent cannot be applied to papers intended for manufacturing bank notes, for which high opacity of the layer(s) is not sought on account of the presence of securities which must be visible in transmitted or reflected light, for instance a watermark, registration-mark prints or security threads.
EP 1 783 273 discloses a security paper with improved durability, in which a composition containing a fluorinated resin is applied to the surface. This composition may be mixed with additional compounds. However, this reference faces similar challenges as previously discussed.
Accordingly, what is needed is an improved durability sheet for the purpose of manufacturing bank notes. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.